It is known in the art that multiple-input multiple-output (MIMO) systems can achieve dramatically improved capacity as compared to single antenna, i.e., single antenna to single antenna or multiple antenna to single antenna, systems. It is also known in the art that if information about the short-term state of the channel is fed back to the transmitter, then the throughput of the channel can be improved with respect to an identically configured system but without short-term feedback. However, because in MIMO systems the overall channel is actually made up of multiple channels, with one channel for each transmit antenna and receive antenna pairing, all of which are time-varying, such short-term feedback requires considerable bandwidth, and it is undesirable to dedicate so much bandwidth to feedback. Furthermore, each channel may span multiple coherence bandwidth intervals, where each coherence bandwidth interval is a swath of frequencies that experience the same effect due to the channel as they pass through the channel. The diversity order of a channel is the number of coherence bandwidth intervals spanned by the transmitted signal.
With some particular architectures, the amount of short-term feedback can be reduced with little loss in performance by replacing the information about the state of the channel with indications about the instantaneous data rate supported by each of the transmit antennas. See for example U.S. patent application Ser. No. 09/714,084. Nonetheless, there are instances in which even such feedback will be unacceptable, because it takes up to much of the available bandwidth of the reverse channel particularly when there is a large number of antennas.